When a tooth experiences trauma such as having become infected or having a nerve in the root adversely affected by proximity to a deeply set filling, the body (dental pulp) takes defensive measures to constrict the root canal to isolate it or its defenses may be overcome by the irritant. If the diagnostic tests of the tooth reveal irreversible damage, a root canal or endodontic procedure is indicated.
The well known root canal procedure is initiated by drilling a hole inwardly from the crown of the tooth in line with the canal of interest. Then files and reamers of increasing larger diameters are used to clean out the canal until it presents a wall of clean dentine and is enlarged sufficiently to be filled with an inert material and then sealed. In cleaning out the canal, it is important that the clinician not change the general shape of the canal such as by creating a ledge in it or by changing its curvature since if either of these events occur, there is an increased probability that one of the successively larger and stiffer files used to clean the canal will perforate the root wall. This in most cases requires surgical correction or extraction of the tooth.
The risk of the tooth canal becoming improperly contoured or perforated is reduced somewhat by the use of files constructed of nickel titanium which are more flexible than previous stainless steel files. Even so, with either type of file, there is a tendency for the file to remove more material from the outer curvature of the root wall than the inner curvature of the root wall as a result of the stiffness of the file. This problem is more likely to occur with larger files.
Some clinicians try to overcome this problem of the file not following the root curve by pre-curving 2-4 mm of the tip of the larger and stiffer files. Some clinicians also file away at the cutting flutes along one side of the file. This way they hope to avoid excessive cutting on the concave or outside curve of the canal.